Why you still have to pay for deflect while starcrossing yout unit in heisho and opponent unit outside battlefield

No — you don't pay any Deflect, as long as your friendly unit in the Star-Crossed pair is at Heisho.

Heisho, Shell of the World reads "Players ignore [Deflect] while paying for spells and abilities choosing something here." Since Star-Crossed chooses a unit at Heisho, the ignore applies to the whole cost-payment step for that spell, and ignored abilities are treated as inactive for that procedure (Rule 766). It doesn't matter that the enemy unit is parked at its Base or carries its own Deflect on the card — the ignore isn't scoped to one target only.

This is confirmed directly by FAQ #11783: targeting your own unit at Heisho with Star-Crossed while the enemy unit is at its base means no Deflect is paid. The one requirement is that a unit you choose for the spell is actually at Heisho; if your friendly unit is somewhere else, the ignore doesn't turn on and you'd pay normally.